There are a number of opioid equianalgesic tables with variations in recommended conversion ratios. However, there are several problems with such tables:

• They do not consider incomplete cross-tolerance. Conversion charts are usually derived from single-dose studies in opioid-naive patients with well-defined pain problems such as post-surgery or dental extraction. Such patients are not tolerant to opioids. When we change opioids in palliative patients, they have become tolerant to their current opioid. However, they won't be quite as tolerant to the new one that is being introduced; that is, the tolerance is not completely crossed over (incomplete cross-tolerance). For an opioid-naive patient, 10 mg morphine tends to be equivalent to about 2 mg hydromorphone. However, for the person who has become tolerant to 1000 mg morphine q4h, that tolerance will not be fully expressed with hydromorphone. Instead of tolerating 200 mg hydromorphone, the person may only tolerate 50 – 100 mg, or even much less.
• They do not take into consideration the individual differences in opioid effectiveness. There is increasing recognition of the importance of pharmacogenetic differences in opioid receptors and metabolism. However, it is not possible to predict and accommodate for this at the bedside.
• They can be particularly dangerous in situations of opioid-induced hyperalgesia (which can be seen as part of the spectrum of opioid-induced neurotoxicity). This is a unique situation of very high opioid tolerance and incomplete cross-tolerance, as described above. It is not as rare as might be thought, and is a scenario in which opioid rotation takes place. In such circumstances, it is felt that active opioid metabolites (morphine-3-glucuronide and hydromorphone-3-glucuronide) accumulate and cause generalized hyperalgesia as well as myoclonus and delirium. In response to the hyperalgesia, the opioid is often increased, which only escalates the problem. We have seen a patient have a morphine infusion increased from 5 mg/h to 700 mg/h over three days, in a futile effort to treat pain that the morphine itself was actually causing (through its metabolite). It is impossible to estimate the actual opioid needs in such patients when converting to an alternative, and if the equianalgesic tables are used, the patient is likely to be seriously harmed.
  

Recent publications[1,2,3,4,5] question the safety and role of conversion tables, with one stating that the use of dose conversion ratios found in equianalgesic tables may be an important contributor to the increasing incidence of opioid-related fatalities.[1]

The “new paradigm” described by Webster and Fine[2] involves opioid rotation without conversion tables. It is a slow process, and the assumption is that there is not a clinically urgent need to switch, as would be seen with opioid-induced neurotoxicity. Their process is as follows[2]:

1. Begin a downward titration of the original opioid by reducing the current dose by about 10 – 30% while beginning the new opioid at a dose that would normally be used in an opioid-naive patient or at the lowest available dose for the formulation.
2. Slowly reduce the dose of the original total daily opioid dose by about 10 – 25% per week while increasing the dose of the new daily opioid dose by about 10 – 20% based on clinical need and safety. In most instances, the complete switch can occur within three to four weeks.
3. Provide sufficient immediate-release opioid throughout the rotation to prevent withdrawal and/or increased pain if the dosing changes prove insufficient. This minimizes the risk of the patient self-medicating due to inadequate relief, which can be fatal.
   

A recent publication has examined the reliability of smart-phone apps for opioid conversion, and concluded that there are significant concerns with regard to the reliability of information provided by apps offering opioid dose conversion, with lack of information regarding evidence-based content and peer review in many cases. It is crucial that better regulation of medical apps is instigated in order to ensure that patient safety is maintained.[6]

So, with that background, here are some recommendations:

1. There are two main risks in opioid conversion: (1) you won't give enough and (2) you'll give too much. In addressing the possibility of under-dosing, aggressive breakthrough doses are your patient's friend. Similarly, patient/family education about what to watch for in case of excessive opioids is important. The correct breakthrough dose is quite simply the one that works and causes acceptable adverse effects (preferably none). Determining the proper breakthrough dose may require empirical titration (i.e., trial and error).
2. In all situations, be mindful of incomplete cross-tolerance. Consider decreasing the dose that you've calculated for the new opioid by about 50%. The longer the patient has been on opioids and the higher the dose (i.e., the more tolerant), the more important this is.
3. There may be times where you don't decrease your calculated conversion dose, but stay with a generous dose of the new opioid. For example, consider a patient who has recently started morphine at 10 mg q4h, and although pain is still poorly controlled, you want to switch to hydromorphone because of nausea. If you use a 1:5 conversion, the hydromorphone dose would be 2 mg q4h. We suggest not cutting that down for incomplete cross-tolerance, since the pain control is poor. This is a low-dose scenario in a patient who has not been on opioids long.
4. When switching opioids due to suspicions of opioid-induced neurotoxicity (hyperalgesia, myoclonus, delirium, usually in the context of renal insufficiency and rapidly escalating opioid dose), then determining opioid equivalences becomes even more imprecise. It may be that 90% of the current opioid dose is directed at treating pain that the opioid itself is causing. The management typically involves discontinuing the offending opioid, hydrating aggressively, and using prn doses of an alternate opioid until baseline opioid needs can be estimated.
5. For any conversion to the transdermal fentanyl patch, use the conversions recommended in the product monograph, such as this one: http://www.janssen.ca/product/114.
6. Consider using a 1:5 ratio between hydromorphone:morphine as a starting calculation, and then work from there with the above considerations in mind.
7. Webster and Fine[1,2,4] suggest that in the absence of a clinically urgent situation, it is likely safest to undertake a gradual conversion.
8. With methadone, you'll need advice of a palliative or pain management expert to review the unique considerations in each case.
   

References

1. Webster LR, Fine PG. Review and critique of opioid rotation practices and associated risks of toxicity. Pain Med. 2012;13:562-570.

2. Webster LR, Fine PG. Overdose deaths demand a new paradigm for opioid rotation. Pain Med. 2012;13:571-574.

3. Passik SD. Opioid rotation: what is the rush? Pain Med. 2012;13:487-488.

4. Webster LR, Fine PG. Review and critique of opioid rotation practices and associated risks of toxicity. Pain Med. 2012;13:562-570.

5. Shaheen PE, Walsh D, Lasheen W, et al. Opioid equianalgesic tables: are they all equally dangerous? J Pain Symptom Manage. 2009;38:409-417.

6. Haffey F, Brady RR, Maxwell S. A comparison of the reliability of smartphone apps for opioid conversion. Drug Saf. 2013;36:111-117.